TAV / PIRATA-17 Meeting, Kiel, Germany September 11, 2012 TAV / PIRATA-17 Meeting, Kiel, Germany Peter Brandt1, Martin Claus1, Richard J. Greatbatch1, Robert Kopte1, John M. Toole2, William E. Johns3 1GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Germany 2Woods Hole Oceanographic Institution, Woods Hole, USA 3RSMAS/MPO, University of Miami, USA Annual and semi-annual variations of equatorial Atlantic circulation associated with basin mode resonance Peter Brandt - GEOMAR

Forcing of Annual and Semi-annual Waves Bi-annual intrusion of tropical water in the northern Benguela upwelling Equatorial Waveguide Coastal Waveguide Wind stress [N/m2] (zonal along equator, meridional along coast) What is the associated variability of circulation and vertical structure? At the equator? Along the coast? °E °N Sea level anomaly [cm] °E °N Rouault 2012

Equatorial Undercurrent (EUC) Observations during TACE reveal mean EUC structure and seasonal variability Pronounced annual and semi-annual cycle Johns et al. 2014

EUC Seasonal Cycle at 23°W Maximum transport in autumn Semi-annual cycle of core velocity Shallow EUC in spring and deep EUC in autumn How can we explain the seasonal cycle? EUC Core Velocity Black circles: shipboard measurements Red lines: moored observations EUC Transport EUC Core Depth Brandt et al. 2014

Zonal Velocity at Equator, 23°W Moored observations of zonal velocity allow analysis of the vertical structure of seasonal variability

Temporal and Vertical Structure of Zonal Velocity Spectrum shows peaks at annual and semi-annual cycle Peaks are dominated by the 2nd and 4th baroclinic mode Good agreement between observations and model

Dominant Seasonal Variability of Zonal Velocity in GCM Simulation 2nd baroclinic mode, semi-annual cycle 4th baroclinic mode, annual cycle Horizontal pattern show amplitude maximum along the equator and generally westward phase propagation as typically found for linear equatorial basin modes GCM output by courtesy of F. Schwarzkopf, C. Böning

Equatorial Basin Modes January 25, 2013 Tropical Ocean Dynamics - Lecture 11 Cane and Moore (1981) discovered analytical solutions of low-frequency standing equatorial modes composed of equatorial Kelvin and long Rossby waves Period of the gravest basin mode: Applications: Resonance of 2nd baroclinic mode semi-annual cycle in the Indic (Jensen 1993, Han et al. 1999) and Atlantic (Thierry et al. 2004, Ding et al. 2009) Resonance of 1st baroclinic mode intraseasonal variability in the Indic (Han et al. 2005, Fu 2007) EDJ behavior (Johnson and Zhang 2003, d‘Orgeville et al. 2007, Brandt et al. 2011, Greatbatch et al. 2012, Claus et al. 2014) Fu 2007: The semiannual SSH variability is characterized by a basin mode involving Rossby waves and Kelvin waves traveling back and forth in the equatorial Indian Ocean between 10°S and 10°N. However, the interference of these waves with each other masks the appearance of individual Kelvin and Rossby waves, leading to a nodal point (amphidrome) of phase propagation on the equator at the center of the basin. The characteristics of the mode correspond to a resonance of the basin according to theoretical models. For the semiannual period and the size of the basin, the resonance involves the second baroclinic vertical mode of the ocean. Simulation of such resonant equatorial modes using a linear reduced-gravity model Peter Brandt - GEOMAR

Reduced-Gravity Model Rectangular box model, with spatially constant oscillating zonal wind forcing 2nd baroclinic mode, semi-annual cycle 4th baroclinic mode, annual cycle

Reduced-Gravity Model Realistic coast lines, with spatially constant oscillating zonal wind forcing 2nd baroclinic mode, semi-annual cycle 4th baroclinic mode, annual cycle

Reduced-Gravity Model Realistic coast lines and realistic wind forcing 2nd baroclinic mode, semi-annual cycle 4th baroclinic mode, annual cycle

General Circulation Model In general good agreement between basin modes simulated with reduced gravity model and GCM 2nd baroclinic mode, semi-annual cycle 4th baroclinic mode, annual cycle

EUC Transport: Observations vs. Reduced-Gravity Simulations Superposition of the two dominant modes are used to explain characteristics of the observed seasonal cycle of EUC transport across the basin

EUC Characteristics: Observations vs. Reduced-Gravity Simulations Main characteristics of EUC at 23°W can be reproduced

TAV / PIRATA-17 Meeting, Kiel, Germany Summary September 11, 2012 TAV / PIRATA-17 Meeting, Kiel, Germany Equatorial zonal velocity variability dominated by linear equatorial basin modes of different periods/baroclinic modes Wind-forced annual and semi-annual basin modes help to explain seasonal cycle of the EUC semi-annual cycle of core velocity; in particular velocity maximum during boreal spring associated with weak winds vertical migration of the EUC core: shallow in boreal spring, deep in boreal autumn Associated propagation of coastal Kelvin waves results in variability of eastern boundary circulation Peter Brandt - GEOMAR

Outlook: Coastal Waveguide EUC measurements are continued in the frame of PIRATA, SFB 754, and EU PREFACE Measurements at 11°S off Angola in the frame of BMBF SACUS/EU PREFACE, next cruise: Oct. 2015

Vertical structure functions for the 2nd baroclinic mode and the 4th baroclinic mode